1 How Mail Works

On the internet, mail is sent from mail host to mail host using the
simple mail transfer protocol (SMTP). To send and receive mail, you
must get it from and send it to a mail host. Every mail host runs a
mail transfer agent (MTA) such as Exim that accepts mails and passes
them on. The communication between a mail host and other clients does
not necessarily involve SMTP, however. Here is short overview of what
is involved.

The mail program—also called a mail user agent (MUA)—usually
sends outgoing mail to a mail host. When your computer is
permanently connected to the internet, it might even be a mail host
itself. In this case, the MUA will pipe mail to the
/usr/lib/sendmail application. It will take care of your mail
and pass it on to the next mail host.

When you are only connected to the internet from time to time, your
internet service provider (ISP) has probably told you which mail host
to use. You must configure your MUA to use that mail host. Since you
are reading this manual, you probably want to configure Emacs to use
SMTP to send mail to that mail host. More on that in the next
section.

Things are different when reading mail. The mail host responsible
for your mail keeps it in a file somewhere. The messages get into the
file by way of a mail delivery agent (MDA) such as procmail. These
delivery agents often allow you to filter and munge your mails before
you get to see it. When your computer is that mail host, this file is
called a spool, and sometimes located in the directory
/var/spool/mail/. All your MUA has to do is read mail from the
spool, then.

When your computer is not always connected to the internet, you
must get the mail from the remote mail host using a protocol such as
POP3 or IMAP. POP3 essentially downloads all your mail from the mail
host to your computer. The mail is stored in some file on your
computer, and again, all your MUA has to do is read mail from the
spool.

When you read mail from various machines, downloading mail from the
mail host to your current machine is not convenient. In that case,
you will probably want to use the IMAP protocol. Your mail is kept on
the mail host, and you can read it while you are connected via IMAP to
the mail host.

So how does reading mail via the web work, you ask. In that case,
the web interface just allows you to remote-control a MUA on the web
host. Whether the web host is also a mail host, and how all the
pieces interact is completely irrelevant. You usually cannot use
Emacs to read mail via the web, unless you use software that parses
the ever-changing HTML of the web interface.